[AVR] Do not chain stores in call frame setup

Previously, AVRTargetLowering::LowerCall attempted to keep stack stores
in order with chains. Perhaps this worked in the past, but it does not
work now: it appears that the SelectionDAG legalization phase removes
these chains. Therefore, I've removed these chains entirely to match
X86 (which, similar to AVR, also prefers to use push instructions over
stack-relative stores to set up a call frame). With this change, all the
stack stores are in a somewhat reasonable order.

Differential Revision: https://reviews.llvm.org/D97853